FIG. 1 is a perspective view of a pressure sensor 2 used to measure a pressurized liquid or gas. The sensor 2 is comprised of an injection-molded plastic housing 4 having two attachment flanges 6. Through-holes 8 in the flanges 6 receive attachment screws, not shown, which allow the pressure sensor 6 to be attached to a surface, such as an engine manifold. A cover 10 is ultrasonically welded or slygard bonded over a cavity 16, which encloses a diaphragm-type pressure sensor element 14 for the pressure sensing. For differential pressure sensing, pressurized gases and/or liquids flow through a hollow port 51 on the backside of the housing 4 (See FIG. 9) and reach the backside of the diaphragm of the pressure sensor element 14 while ambient gases, typically air surrounding the sensor 2, flow through the opening 9 in the cover 10 and reach the topside of the diaphragm of the pressure sensor element 14. The two pressures, i.e., in the hollow port 51 and ambient pressure, exert forces on the diaphragm of the pressure sensor element 14 mounted in the cavity 16 and cause diaphragm stresses and diaphragm deformation.
The diaphragm-type pressure sensor element 14 includes a piezoresistive transducer, the resistance of which changes in response to diaphragm deflection caused by pressure applied to the diaphragm. The piezoresistive element's resistance changes are converted into measurable electrical signals by circuitry in an integrated circuit (IC) 18 co-located within the cavity 16 and which is connected to the sensor 14 via lead wires 20 that extend between the sensor 14 and the IC 18. Electrical signals generated by the IC 18, and which represent a pressure applied to the sensor 14 are carried from the IC 18 to a lead frame not shown in the figure via lead wires 20 that extend from the IC 18 to a lead frame 21 inside the cavity but not visible in FIG. 1.
The lead wires 20 used to connect the sensor 14 to the IC 18, and which are used to connect the IC 18 to the lead frame 21, are thin. Regardless of how the lead wires 20 are connected to the lead frame 21, the attachment of the lead wires 20 to the lead frame 21 is susceptible to failure if the lead wires 20 are subjected to mechanical stresses.
Inasmuch as the pressure sensor 2 is used to measure pressures of liquids and gases that are known to have corrosive chemicals in them, the cavity 16 is substantially filled with a gel 22, not visible in FIG. 1 and which covers the sensor 14, the IC 18, the lead wires 20 and the lead frame 21. The gel 22 acts to protect the devices and connections inside the cavity 16 from corrosive chemicals in liquids and gases, the differential pressure of which in port 51 is being measured.
While the gel 22 is effective in protecting electronic devices and connections from chemicals, the gel 22 is also effective in transmitting throughout the cavity 16, vibration and shock waves that the sensor 2 might be subjected to, especially when the sensor 2 is used to sense the various pressures commonly found in motor vehicles. Harmonic vibration and random vibration are usually present in a motor vehicle under standard operating conditions. The high frequency vibration of a motor vehicle causes the gel 22 to vibrate within the cavity 16. In some circumstances, sudden loading or impact within, or to the vehicle, causes vibration waves of much greater amplitude than those present during normal operation. In other words, relatively low amplitude harmonic vibration waves occur in a vehicle under normal operating conditions, while high amplitude shock waves are a more random occurrence. Road surface irregularities, engine vibration and door closures are just three sources of impacts and vibrations that can create waves in the gel 22 that cause connection failure between the lead wires and IC 18, sensor 14 and lead frame 21. Wave fronts that are induced within the cavity propagate through the gel 22 and strike the lead wires causing them to break. These waves apply normal or near normal forces on the lead wires causing a combination of tensile and bending stress on the lead wires and their bond to the substrate. Two modes of failure occur in the bond between the lead wires and substrate as well as the lead wires themselves. Harmonic vibration and random vibration create completely reversed cyclic loading thereby fatiguing the bonding material between the lead wires and the substrate. Under fatigue loading the bond between the lead wires and the substrate exceeds bonding material's endurance limit and failure of the connection occurs. Failure of the bond between the lead wires and the substrate is also caused by shock loading. In the case of shock loading, fracture stress of the bonding material is exceeded and causes connection failure to occur.
FIG. 2 is a top view of the cavity 16. Reference numeral 30 identifies wave fronts that are induced by either shock, vibration or both. Since a shock wave is in many respects the same as a vibration wave, for purposes of brevity, such wave fronts are considered hereinafter to be vibration-induced wave fronts. The vibration-induced wave fronts are depicted in FIG. 2 as originating from the right-hand side of the cavity 16 and as traveling toward the left-hand side and thereafter, back and forth. Reference numerals 32 and 32B, hereafter collectively referred to as “32” identify reflected and/or refracted wave fronts. As set forth above, reference numeral 20 identifies lead wires that connect the sensor element 14 to the integrated circuit 18 and which also connect the integrated circuit 18 to lead frame 21. The lead frame 21 passes through the sidewall 28 of the cavity 16 where connections are made to the socket, not shown in the figures.
The reflected/refracted wave fronts 32 are shown in FIG. 2 as impinging upon the lead wires 20-1 orthogonal to, or nearly orthogonal to the wires 20-1. When wave fronts (30 or 32) in the gel 22 impact the wires, they exert a lateral force on the wire that is proportional to the gel pressure at the wave front. A lateral force is thus exerted on the wires, the magnitude of which is equal to the product of the wave front pressure and the area of the wire to which the wave front pressure is applied. As set forth below, the wave fronts 30 and 32 that strike the lead wires 20 and 20-1 at right angles or approximate right angles therefore tend to exert lateral forces, which over time, fracture the lead wires and/or their attachment to the integrated circuit 18, the sensor 14 and/or lead frame 21.
FIGS. 3A and 3B are graphical depictions of the forces exerted on a “long” lead wire 20A and a “short” lead wire 20B. Lateral forces from the wave fronts (30 or 32) are distributed over the length of the wire and represented in the figures by the arrows identified by “F.” While the wave fronts (30 or 32) can strike the wires at any angle, the force that is orthogonal to the wire's axial length is the force that tends to break the wire and/or its bond due to the lateral displacements D1 and D2 that a force normal to the wire's axis tends to cause.
As is well known, the total force F exerted on a surface of area A, by a pressure of magnitude P acting uniformly over the entire area, is the product of P and A. In other words,F=P×A where F is the force on an area A under a uniform pressure P.
In the cavity 16, since the gel edge is considered herein to be essentially “anchored” to the sidewall, the wave front pressure P is proportional to the gel acceleration a multiplied by the gel density ρ times the “length” of the gel, which is the width of the cavity 16. Stated another way,P∝ρ×a×Lwhere ρ is the density of the gel 22, a is the acceleration of the gel and L is the “length” of the gel, (i.e., the length or width of the cavity 16).
FIGS. 3A and 3B show that for a given wave-front pressure, the total force exerted on a “long” lead wire will be greater than the total force exerted on a “short” lead wire. The wave fronts 30 and 32 that strike long lead wires thus tend to cause such wires and/or their connections to fail. A method and/or apparatus to reduce vibration-induced wave fronts in a cavity 16 containing gel 22 and/or redirect such wave fronts would be an improvement over the prior art in that reducing or redirecting wave fronts would tend to reduce lead wire failure as well as reduce lead wire connection failure.